Austenitic heat-resistant alloy having excellent strength at high temperatures is favorably used for heat-resistant castings, such as reactor tubes for producing ethylene, which are exposed to high temperature atmosphere for a prolonged period of time.
During use in high temperature atmosphere, a metal oxide layer is formed over the surface of austenitic heat-resistant alloy, and the layer serves as a barrier for giving sustained heat resistance to the material, whereby the material can be protected from high ambient temperatures.
However, when the metal oxide is Cr-oxides (consisting mainly of Cr2O3), the oxide layer is low in density and deficient in tight adhesion and therefore has the problem of being prone to spall off during repeated cycles of heating and cooling. Even if remaining unseparated, the layer fails to sufficiently function to prevent penetration of oxygen and carbon from the outside atmosphere, exhibiting the drawback of permitting the internal oxidation or carburization of the material.
In this regard, the following patent literature has been proposed in connection with austenitic heat-resistant alloys which are adjusted in components and composition to ensure the formation of an oxide layer comprising mainly of alumina (Al2O3) having high density and resistant to the penetration of oxygen and carbon.
Patent Literature I: JP Unexamined Patent Publication SHO52-78612
Patent Literature 2: JP Unexamined Patent Publication SHO 57-39159
These disclosures of Patent Literature are adapted to form over the surface of the material an oxide layer consisting mainly of Al2O3 by giving a higher Al content than in common austenitic heat-resistant alloys.
Patent Literature 1 proposes an Al content of over 4% and Patent Literature 2 an Al content of at least 4.5% in order to form an Al2O3 layer of sufficient thickness which is prevented from spalling off during use at high temperatures.
Al is a ferrite forming element, and accordingly an increased Al content impairs the ductility of the material to result in decreased strength at high temperatures. This tendency toward decreased ductility is observed when the Al content increases over 4%.
Accordingly, the austenitic heat-resistant alloys of the foregoing literature have the drawbacks of exhibiting impaired ductility although improved barrier function in high temperature atmosphere is expectable as afforded by the Al2O3 layer.